A heterodyne receiving system has been known, that receives spectra from astronomical objects and atmospheric minor molecules.
For example, it is on its way to becoming assured as a result of recent research efforts that rarefied interstellar materials including gases and dusts give a birth of a star; and the rarefied interstellar materials emit electromagnetic waves within a millimeter or submillimeter band of oscillation frequencies fixed by every molecular species associated with rotating motions of molecules of the interstellar materials. These electromagnetic waves are observed so as to achieve research outcomes with respect to birth and evolution of stars. Known as equipment for observing the electromagnetic waves are radio telescopes.
To observe, for example, ozone spectral emission by using a superconductor insulator superconductor (SIS), a system for measuring ozone in the atmosphere has been known.
The electromagnetic waves of the astronomical objects and the atmospheric molecules are extremely imperceptible and need to be amplified. It, however, is not easy to amplify the electromagnetic waves because noise from a receiver reaches a considerable level for signal processing. In addition, amplification equipment is rarely available, that may directly amplify the waves at frequency bands called millimeter and submillimeter bands. Accordingly, a local oscillator signal (or an LO signal) that is slightly different in frequency from an electromagnetic wave signal (a radio-frequency signal or an RF signal) is generated; and the electromagnetic wave signal and the local oscillator signal are mixed together to obtain a difference of these frequencies. This enables the signal within a high-frequency band to be converted to an easily-processable intermediate-frequency signal (IF signal) within a low-frequency band. This is what has been referred to as the heterodyne receiving system described above, and a frequency converter used for this system is referred to as a mixer.
The heterodyne receiving system using the mixer outputs two frequency bands—high-frequency waves and low-frequency waves—where a frequency of the LO signal (fLO) is configured to be a center of the IF signals. This IF band is referred to as a sideband; and in particular a sideband on the side of the high-frequency waves is referred to as an upper sideband (USB) while a sideband on the side of the low-frequency waves is referred to as a lower sideband (LSB) (see FIG. 16). Two IF bands are obtained as exactly same frequency from the mixer. This is referred to as a double sideband (DSB) mode. To observe the waves independently, the IF bands are separated into USB and LSB. This is referred to as a single-sideband (SSB) mode.
The observation is carried out by using one of a USB signal and an LSB signal—for example, only the USB signal is used. In this case, the LSB band is eliminated by using a filter. The signal passed through the filter is amplified and is observed. The band that should be observed (e.g., USB) is referred to as a signal band, and the band that should be eliminated (e.g., LSB) is referred to as an image band.
To observe the spectra from the astronomical objects and the atmospheric molecules efficiently, it is necessary to sufficiently eliminate noise and interference of the image band from the signal that should be observed. Since spectral intensity of the astronomical objects changes according to a sideband ratio (a sensitivity ratio between the signal band and the image band) of the receiver, the single-sideband receiver is greatly significant for an accurate measurement of the spectra. Generally known as the single-sideband receiver is a quasi-optical receiver using an interferometric Martin-Puplett-type frequency filter with use of optical elements.
Also known as the single-sideband receiver is a receiver using a band rejection filter to eliminate an image band from a heterodyne receiver (see, for example, Patent Document 1 and Non-Patent Document 1). Such a band rejection filter for eliminating the image band is also referred to as an image rejection filter. Patent Document 1 discloses a system (a 2-backshort) for separating a double sideband by balanced two superconductor mixers.
The image rejection filter is one of the essential components to eliminate the unnecessary signals or to suppress the electromagnetic interference. Several image rejection filters have been developed, that use a waveguide. The waveguide is one of transmission lines used in the millimeter and the submillimeter bands.
FIG. 17 illustrates explanatory drawings indicating an example of a structure of a traditional waveguide-type image rejection filter disclosed in Non-Patent Document 1. FIG. 17 (a) illustrates a perspective view of a main part, and FIG. 17 (b) indicates an example of measurements of the main part. The measurements are indicated in units of millimeters (mm). As illustrated in FIG. 17, the traditional waveguide-type image rejection filter 100 comprises a main track 101 whose wide-width surface has resonators 103 placed thereon at intervals of about three-fourths intertubular wavelength, the resonators functioning as waveguides having a length of a one-half intertubular wavelength. The main track 101 is connected to each resonator 103 through an iris 105. The main track 101 is 2.54 mm in width and the iris 105 is 1.10 mm in opening width, in a direction toward the back of the main track illustrated in FIG. 17 (b).
FIG. 18 indicates a figuration of one of two waveguide blocks constituting the image rejection filter of FIG. 17. In this drawing, reference numerals 101, 103 and 105 point components comparable to the main track 101, the resonator 103 and the iris 105 of FIG. 17 (b), respectively.
FIG. 19 is a block diagram indicating a configuration example of an ozone measurement system using the image rejection filter of FIG. 17 (see Non-Patent Document 1). This ozone measurement system increases an image signal elimination ratio by placing the image rejection filter of FIG. 17 between a feedhorn and an SIS mixer.